Parallelogram type wheelchair lifts are offered by a number of manufacturers, including The Braun Corporation of Winamac, Ind. in its L900 series of lifts, as shown in its U.S. Pat. No. 5,261,779, and by Ricon Corporation of Pacoima, Calif. in its S-series of 30 lifts, as shown in U.S. Pat. No. 4,534,450 and expired Re 31,178. These lifts employ various mechanisms to cause the platform to move arcuately upward from the horizontal transfer level to a vertical or over-vertical stowage position. One system involves the use of an articulated lever assembly comprising a pair of arms of unequal length pivotably connected to each other at one end, and pivotably connected at their other ends respectively to: a) the vertical lift arm end link, at the bottom end of which is pivotally secured the platform, and b) the inboard end of the platform. As the hydraulic ram in the lifting assembly is actuated, lifting the platform from the ground level toward the transfer level, a sliding block, pivotally secured at the common center of the two arms, comes into contact with the lower arm of the parallelogram. As the lifting continues and the end link approaches the lower arm, the lower longer arm of the lever assembly is pushed downwardly. In turn this causes the outboard end of platform to rotate upwardly to the stowed position.
To prevent platform free fall, a number of strategies are employed as set forth U.S. Pat. No. 5,806,623 issued Sep. 15, 1998, the disclosure of which is hereby incorporated by reference. These strategies include stud and slot arrangements of the Braun Model L211U, Ricon's Saucier U.S. Pat. No. 5,605,431 (FIGS. 13-15) and a diagonal spring arrangement across the arms of the articulated lever arm assembly as set forth in the aforesaid U.S. Pat. No. 5,806,632.
The outboard end of the platform typically includes a roll stop safety barrier. A variety of actuation strategies are employed, including cables, chains and levers, with or without gas spring or linear actuator assist. Likewise the inboard end of lift platforms are provided with a variety of strategies for actuating inboard barriers. An example is a cam actuated cable system of Saucier, et al., U.S. Pat. No. 5,605,431 (1997) which was commercially available at least as early as Mar. 16, 1992 as the Ricon Model S 5003. This system employs a bell crank and cable. In that system, the lifting parallelogram actuates a cable, the length of which is controlled by a cam assembly pivoted to the lifting end link or an arm of the parallelogram so that as the platform moves, an interior barrier is raised or lowered by the other end of the cable. The articulated lever arm anti-free-fall assembly is not involved in the inboard barrier actuation.
Cable systems however have a number of serious drawbacks, among them being that the cable is difficult to adjust precisely, thereby requiring frequent readjustments, as it stretches in use and tends to lengthen or shorten with temperature. In addition, a cable can fray or break in use, and has limited strength. The barrier position varies under all these conditions and can become out of synchrony with the platform position. In some cases the barrier could prematurely descend to a near-horizontal position prior to the platform reaching the transfer level, in which case it could impact the side of the vehicle or the sill lip at the entry causing damage to the lift and/or vehicle.
Accordingly, there is a need for an improved positive inboard barrier actuation system that does not have the drawbacks of such cable systems.